9vDC to 5v and 3.3v DC Converter design

Hi,

I need to design a DC to DC convertor from 9vDC to 5vdc and 3.3vDC.

Because the micro-controller which i am using requires 3.3v MAX.

But i am using some active sensors which requires more then 3.5v DC .

So i planed to design a DC to DC converter with 9vDC.

I goggled and found that LM7805 can be used for convert the 9v to 5v.


Is there any IC would take 9v as input and produce both 3.3v and 5v both.

after goggled i found some circuits and figured as attached.



whether ihe IC which indicated in the figure are ok.

I could not understand upto what extend they could give the Current whether in A or in mA

Battery:

Can i use a single 9v battery or 6AA battery's in series.

please tell me which would give more battery backup , because i am going to use this in WIRELESS SENSOR NETWORKS design .

thanking you

Do you know how much current you need on each power line? There are dual output regulators, but I only know of ones that are either low current (like the MIC5211-3.3/5.0BM6 from Micrel, around 80mA) or more complicated than using two separate ones.

The 7805 is good for up to 1 ampere, the LD1086 for 1.5 ampere. But, if you are drawing that through them, they will get hot and need heatsinks. Your battery won't last long at that current either.

The 7805 needs at least about 7.5V input. If you use normal cells, like alkaline, at 9V then their voltage will drop below that long before they are really used up, especially if they get cold. A small 9V battery (PP3) will have much less capacity than 6x AA cells, so I would go for the AA's. NiMH would possibly be better - they hold their voltage better until nearly empty.

Also due to the above, consider a low-dropout 5V regulator, like: LT1129CT-5 (700mA - also has built-in reverse battery protection so you can omit the diode which is losing you 0.6V) or LP2954 (250mA). If you used a low dropout 5V like those, you could use 6x NiMH AA cells for 7.2 volt input.

Remember that the 5V regulator will also have the current for the 3.3V circuit, if you wire them as shown.

Knowing your current requirements will help us give you more accurate answers...

A buck/boost converter supports longer battery life time since, the linear regulators just convert the 9V to 5V and 9V to 3,3V to heat. Application like this: **broken link removed** .
Depending on the current demand you might take a look at the iC-DC (https://ichaus.biz/product/iC-DC ) with a hybrid converter architekture and two adjustable output voltages.

Enjoy your design work!

thanku for ur replays

@FoxyRick


The micro-controller which i am using has following specifications. the micro-controller has both MCU+RF module with in it .

Supply Voltage= 2.7 - 3.6 V
Transmit Current (Typ@3.3V) =80mA
Idle/Receive Current (Typ@3.3V) =20mA
Power-down Current (Typ@3.3V) =1.6uA


Regarding sensors
At present i am using two sensors
Sensor-1 specifications:
Voltage = 3v to 5v
current = o.5 to 2.5mA
Sensor-2 specifications:
Voltage= 3.3 to 20v
current= <7mA

I was confused in calculating the over all power consumption .

please help me in that too.

thank you

---------- Post added at 10:53 ---------- Previous post was at 10:42 ----------

@HTA

A buck/boost converter supports longer battery life time since, the linear regulators just convert the 9V to 5V and 9V to 3,3V to heat. Application like this:

Click to expand...

I gone through the links which u had attached it is a good product but As i am from India i need to ship it, although the product cost is less i need to pay for shipping charges too due to which i am in a dynamo how to use this particular product.

But still i did not enquirer it in the market i shall do and i will let you know about it too.

thanking you

Those current requirements are quite easy, and your software can make a great saving too if well written. If your design requirements allow it, then the MCU can power down for most of the time, just powering up from an internal timer for the sensor read and transmit. Also, you might be able to turn off the sensor power supply if they have a quick start-up time, and you rewire the power supply so that the 3.3V regulator runs from the battery as well. Anyway...

What is your transmitting:not-transmitting ratio? That will give you the average current. Let's say for argument that it's 25%. Let's also say that for 50% of the time the MCU will be put into power down mode by your software, waiting to be awakened by a software timer for the next data burst. You might have other things that need some current too though, for example pull-up resistors etc. I'll leave those to you.

So, 50% at 1.6µA, 25% at 20mA and 25% at 80mA. On average then we have a current consumption of 0.5*0.0016 + 0.25*20 + 0.25*80 = 25mA. That's just for the MCU. We still need a regulator capable of 80mA (at least) peak current, but for your power dissipation and battery life calculations the average is only 25mA.

For the sensors, without more information on possible power-down then we need to look at the worst case. We'll call that a total of 10mA on the 5V line.

Wired the way you have it then, the 5V regulator, which also passes the current for the 3.3V regulator, will have 90mA peak and 35mA average going through it. The 3.3V regulator will just have its own 80mA peak and 25mA average. You have a very wide choice of regulators with current requirements being quite low.

I would still very much advise a low-dropout regulator for the 5V, and you need one also (as you already have in the diagram) for the 3.3V.

Battery life: Alkaline cells have a greater capacity than NiMH, but their voltage drops off continually throughout use so they often 'fail' before NiMH in a circuit that needs a certain voltage. NiMH keeps almost a constant 1.2V until almost exhausted. Given 2000mAh for an average AA NiMH cell, then with the averages that we have calculated above, an NiMH battery will last for 2000/35 = 57 hours.

Not long!

If you transmit requirements allow it though, you could make far greater savings on current by running the MCU at power down for most of the time, like 90%, and just waking up periodically to do a sensor-read and transmit.

You could even look at using a 5V regulator with an on/off control input, or using a MOSFET switch to turn the 5V rail on and off. Then, if you connect the 3.3V regulator directly to the battery so that the MCU always has power, you can turn off the 5V rail to the sensors to save their power during power-down mode. That depends on their start-up time as well though.

also, a switching regulator as suggested would be a good improvement giving perhaps 50% extra battery life (with 7.2V NiMH stack) at a quick guess; more if you use a higher voltage battery.

I hope some of that helps...

@FoxyRick
First of all i Thank you for your detailed explanation's. i learned a lot from that and i thank you again.....

I would still very much advise a low-dropout regulator for the 5V, and you need one also (as you already have in the diagram) for the 3.3V.

Click to expand...

Then which regulators would be preferable for 5v and 3.3v . what should be the amperes of those.

while goggling i found a seller who had mentioned about 77different types LDR here it is

**broken link removed**

As you had suggested i would select 6_AA_Alkaline battery's (Duracell made).

i made small change in the previous circuitry before i took 5v as input to 3.3v regulator but now i am giving both the regulators with same voltage 9v parallely . would it improve out currents of the regulators.

i hope if i connect it in parallel i can now have a control on 5v regulator through micro-controller.

You could even look at using a 5V regulator with an on/off control input, or using a MOSFET switch to turn the 5V rail on and off.

Click to expand...

could you please suggest which MOSFET should i use as a ON /OFF switch to control 5v regulator.

i still did not fill the names for the LDR's i will do it based on ur suggest.

thanku